Include a README in binary firmware .zip.

Fixed #234.
If no explicit port given to upload_hex.sh, use SERIAL_PORT.

This matches where the Makefile looks for the port when flashing.
Compile binaries with DEBUG=0.
Set PLATFORM before 'make clean' - it only cleans for current platform.
Update changelog and versions for v6.0.1 release.
Bump to next dev. release.